1. Technical Field
The present invention relates to a thermostat malfunction detection device that detects a stuck open state of a thermostat that opens and closes a cooling water flow channel between a water-cooled engine and a radiator, and particularly relates to a thermostat malfunction detection device that improves malfunction detection accuracy.
2. Related Art
In a water-cooled engine that is mounted on a vehicle such as a passenger vehicle for example, cooling water that cools a cylinder head, a cylinder liner, and the like of an engine is circulated in a radiator which is a heat exchanger, and is cooled by heat exchange with traveling wind. Also, this engine includes a thermostat that closes a flow channel to stop water supply to the radiator when cooling water temperature is equal to or lower than a predetermined temperature, for the purpose of warming up air after cold start. When the thermostat malfunctions to be stuck in an open state, a time taken to warm up the air becomes long excessively, and the malfunction becomes a cause of a trouble that is referred to as overcool in which the cooling water temperature is operated by a lower temperature than a usual water temperature.
For example, WO2013/168529A1 describes calculating a malfunction-time highest water temperature and a normal-time lowest water temperature on the basis of an engine speed, an engine load, a vehicle speed, an outside air temperature, etc. and comparing these calculated values with an actual engine water temperature, as a past technology relevant to malfunction detection of the thermostat.
In recent years, an active grill shutter is proposed which reduces air resistance of the vehicle by closing a part of a grill (a duct) that introduces traveling wind from outside the vehicle into a radiator by means of a shutter in order to improve fuel efficiency, when a cooling request is not severe comparatively. The shutter is normally closed while warming up the engine in the vehicle that includes this active grill shutter, and thus a passing air amount of the radiator is reduced. In this case, even when the malfunction of thermostat stuck open occurs, a water temperature difference from a normal time becomes smaller, and thus the malfunction is unable to be detected in some cases.